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neuromuscular function and locomotion: a military-based study
Jordane G. Grenier, Guillaume Millet, Nicolas Peyrot, Pierre Samozino, Roger Oullion, Laurent Messonnier, Jean-Benoît Morin
To cite this version:
Jordane G. Grenier, Guillaume Millet, Nicolas Peyrot, Pierre Samozino, Roger Oullion, et al.. Effects of extreme-duration heavy load carriage on neuromuscular function and locomotion: a military-based study. PLoS ONE, Public Library of Science, 2012, 7 (8), pp.e43586. �10.1371/journal.pone.0043586�.
�hal-01232323�
Neuromuscular Function and Locomotion: A Military- Based Study
Jordane G. Grenier
1,2*, Guillaume Y. Millet
1, Nicolas Peyrot
3, Pierre Samozino
4, Roger Oullion
5, Laurent Messonnier
4, Jean-Benoıˆt Morin
11
Laboratory of Exercise Physiology (EA4338), University of Lyon, Saint-Etienne, France,
2Safran Group, Sagem, Land Warfare, Massy, France,
3University of La Reunion, UFR SHE, CURAPS-DIMPS (EA4075), Reunion Island, France,
4Laboratory of Exercise Physiology (EA4338), University of Savoie, Le Bourget du Lac, France,
5Sports Medicine and Myology Unit, Bellevue Hospital, University Hospital Center, Saint-Etienne, France
Abstract
Trekking and military missions generally consist of carrying heavy loads for extreme durations. These factors have been separately shown to be sources of neuromuscular (NM) fatigue and locomotor alterations. However, the question of their combined effects remains unresolved, and addressing this issue required a representative context.
Purpose: The aim was to investigate the effects of extreme-duration heavy load carriage on NM function and walking characteristics.
Methods: Ten experienced infantrymen performed a 21-h simulated military mission (SMM) in a middle-mountain environment with equipment weighing ,27 kg during battles and ,43 kg during marches. NM function was evaluated for knee extensors (KE) and plantar flexors (PF) pre- and immediately post-SMM using isometric maximal voluntary contraction (MVC) measurement, neural electrical stimulation and surface EMG. The twitch-interpolation method was used to assess central fatigue. Peripheral changes were examined by stimulating the muscle in the relaxed state. The energy cost, mechanical work and spatio-temporal pattern of walking were also evaluated pre2/post-SMM on an instrumented treadmill in three equipment conditions: Sportswear, Battle and March.
Results: After the SMM, MVC declined by 210.263.6% for KE (P,0.01) and 210.7616.1% for PF (P = 0.06). The origin of fatigue was essentially peripheral for both muscle groups. A trend toward low-frequency fatigue was detected for KE (5.5%, P = 0.08). These moderate NM alterations were concomitant with a large increase in perceived fatigue from pre- (rating of 8.362.2) to post-SMM (15.962.1, P,0.01). The SMM-related fatigue did not alter walking energetics or mechanics, and the different equipment carried on the treadmill did not interact with this fatigue either.
Conclusion: this study reports the first data on physiological and biomechanical consequences of extreme-duration heavy load carriage. Unexpectedly, NM function alterations due to the 21-h SMM were moderate and did not alter walking characteristics.
Clinical Trial Registration: Name: Effect of prolonged military exercises with high load carriage on neuromuscular fatigue and physiological/biomechanical responses. Number: NCT01127191.
Citation:
Grenier JG, Millet GY, Peyrot N, Samozino P, Oullion R, et al. (2012) Effects of Extreme-Duration Heavy Load Carriage on Neuromuscular Function and Locomotion: A Military-Based Study. PLoS ONE 7(8): e43586. doi:10.1371/journal.pone.0043586
Editor:
Alejandro Lucia, Universidad Europea de Madrid, Spain
Received
March 13, 2012;
AcceptedJuly 23, 2012;
PublishedAugust 22, 2012
Copyright:ß
2012 Grenier et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding:
No current external funding sources for this study.
Competing Interests:
The authors have the following interests. Jordane G. Grenier is currently PhD student at University of Saint-Etienne and is also affiliated to the Safran group via an industrial PhD thesis contract. As part of this thesis contract, Jordane G. Grenier’s work aims to evaluate the physiological consequences of the FELIN land warfare system (developed by the Safran group and used in the present study as the equipment to be carried by the subjects). There are no further patents, products in development or marketed products to declare. This does not alter the authors’ adherence to all the PLoS ONE policies on sharing data and materials, as detailed online in the guide for authors.
* E-mail: [email protected]
Introduction
Outdoor activities requiring self-sufficiency, specific gear and/or involving bivouac (e.g. trekking and military missions) are characterized by the carriage of considerable equipment and supplies. This specific feature implies high-to-severe conditions of locomotion [1,2] and requires high levels of metabolic and
mechanical energy from the carrier, even for short-term walking
[3,4,5,6]. In addition, these walking efforts are generally prolonged
for hours or even days and can be associated with sleep rhythm
disturbance or deprivation, caloric restriction, as well as environ-
mental and psychological stresses, especially in the military context
[7,8,9]. Taken together or separately, these factors have delete-
rious physiological consequences [10,11] and may induce neuro- muscular (NM) fatigue.
NM fatigue is defined as an exercise-related decrease in the maximal strength or power of a muscle, whether or not the task can be sustained [12]. Fatigue potentially involves processes at all levels of the motor pathway from the motor cortex to skeletal muscle [13]. Classically, alterations of neuromuscular function due to fatigue are classified as central (neural) or peripheral (muscular) in origin. Central fatigue corresponds to a failure of the central nervous system to drive the motoneurons adequately and appears with exercise [14] while peripheral fatigue may involve all processes located distal to the neuromuscular junction/sarcolem- ma, in particular the fatigue-induced alterations in excitation–
contraction coupling [15]. Central and peripheral origins are mutually dependent since recruitment of motoneurons depends on the descending drive from supraspinal sites and central drive is controlled through a combination of factors including excitatory and inhibitory reflex inputs from different peripheral afferents.
Fatigue is known to depend on exercise duration and intensity, and type of muscle contraction [16,17].
Previous studies have investigated the effects of exercises with heavy load carriage of short-to-medium duration on NM function.
Clarke et al. [18] have reported decreases in trunk, knee and ankle flexor/extensor maximal voluntary contraction (MVC) in soldiers after ,3-h walking (12.1 km at 4 km.h
21) while carrying loads up to 27 kg. More recently, Blacker et al. [19] showed that the carriage of a 25-kg pack during 2-h treadmill walking induced a 15% loss in knee extensor (KE) MVC and was associated with moderate central and peripheral fatigue. Furthermore, slight low- frequency fatigue (LFF, which has been linked to excitation- contraction coupling failure and muscular damage [20]) was detected in this study for both level and downhill walking [19].
Other studies have investigated the NM consequences of exercises without load carriage (or with light equipment) but of extreme duration. Very large KE and plantar flexor (PF) MVC declines were observed in ultra-marathon runners after a 24-h treadmill run (241% and 230%, respectively [21]) and a ,40-h mountain ultra-marathon (235% and 239%, respectively [22]).
In both studies, MVC declines were associated with large central activation deficits (depending on the muscle group tested) and peripheral fatigue. LFF was also observed after the mountain ultra-marathon, likely due to long downhill sections (,9 500 m of total negative elevation) that involved intense eccentric muscular actions known to induce muscular damage [22,23].
To date, the question of the combined effects of heavy load carriage and exercise of extreme duration on NM fatigue remains however unresolved. Since military efforts naturally and severely combine these two features [9], we thought it justified and relevant to use a military mission context to address this question. Indeed, beyond their scientific novelty, such data obtained in a real-world context could be applied to strategic planning and used in the military theater (e.g. management of forces during actual missions). Therefore, the main purpose of the present study was to investigate the effects of a prolonged mission with heavy military load carriage on the NM fatigue of two major muscle groups involved in human walking and load carriage [24]. For this purpose, the KE and PF NM function of experienced infantrymen was assessed before and after a 21-h simulated military mission (SMM) specifically designed to represent the current operational reality.
In addition, in view of the existing links between NM fatigue, load carriage and the risks of slips, falls or injuries during locomotion [2,24,25,26,27,28], this study design allowed us to investigate the effects of extreme-duration heavy load carriage on
walking mechanics and energetics. Indeed, although acute consequences of load carriage have been widely considered [3,4,5,6], very little is known about the effects of fatigue and its potential interaction with load carriage on walking characteristics.
Moreover, the few studies that have hitherto explored similar issues mostly used fatiguing laboratory protocols (e.g. treadmill run [27,29,30,31]), not real exercises in the field, and this may limit the representativity of their observations [32]. Therefore, the second- ary aim of the present study was to investigate the consequences of this 21-h SMM on the energy cost, mechanical work and spatio- temporal parameters of walking.
Concerning the primary aim of the present study, we hypothesized that both central and peripheral NM function alterations would be larger after the SMM than after loaded- walking exercises of intermediate duration, i.e. from 2 to 3 h [18,19]. This is because of (i) the much longer exercise duration, (ii) the heavier loads carried, and (iii) the additional stressors inherent to the mission-related effort (e.g. sleep rhythm and comfort disturbances, hilly terrain). However, NM function changes of lower magnitude than after ultra-marathon runs [21,22] were expected (especially at the peripheral level) because, contrary to running, walking is not associated with repeated impacts or intense muscular eccentric actions [33,34]. Regarding the secondary aim of this study, we hypothesized that walking characteristics would change after the 21-h SMM, especially the mechanical parameters [30,31] that have been shown to be more discriminating than energetical ones at moderate walking speed [4].
Methods
Subjects and Ethics Statement
Ten males volunteered to participate in this study after being informed about the procedure and risks associated with the protocol. They were all involved in regular physical activities (5.462.7 hours per week) and were not presenting recent muscular, joint or bone disorders or receiving any medication that could interfere with their NM responses or walking pattern or influence their energetic metabolism. The subjects were recently- retired infantrymen (seven from the French Foreign Legion) with a career of 14.168.3 years and had extensive high-load carriage and military experience. Their anthropometrical and physiological characteristics were as follows: age: 38.968.9 yrs, height:
17765 cm, mean leg length: 90.963.6 cm, body mass (BM):
82.969.3 kg, BM index: 26.762.2 kg.m
22, body fat percentage:
19.463.1%, maximal heart rate (HR
max): 190617 bpm, absolute maximal oxygen uptake ( V VO _
2max): 3.5360.36 L.min
21, V VO _
2maxrelative to BM: 42.365.2 mL.kg
21.min
21.
Written informed consent was obtained from the subjects, and the study was conducted according to the Declaration of Helsinki.
The protocol was approved by the local ethics committee (Comite´
de Protection des Personnes, Sud-Est 1, France) and registered at http://clinicaltrial.gov (reference: NCT01127191).
Protocol Overview
The subjects were included in the study one week before the beginning of the specific research protocol. Inclusion sessions consisted of (i) a complete medical examination with anthropo- metric data collection, (ii) a standardized incremental maximal aerobic test, and (iii) a complete familiarization with the different devices/protocols used in the experimentation (see details in the section subject characterization and familiarization below).
The specific research protocol lasted 24 h and consisted of two
(pre/post) 90-min laboratory measurement sessions separated by
the 21-h SMM. Heart rate was the only parameter monitored continuously throughout the protocol using telemetric cardiofre- quencemeters (Polar RS800CX, Polar Electro Oy, Kempele, Finland). Chronologically, the pre-SMM session (PRE) began with an evaluation of the subjects’ instant rating of perceived fatigue (RPF), and a 6-min moderate cycling warm-up immediately followed by a standardized NM function evaluation (see details in the sections relating to fatigue assessment below). After 5 min of rest, expired gases were collected over 10 min of unloaded standing.
Then the subjects performed three 3-min level-walking trials at 4 km.h
21on an instrumented treadmill, during which walking energetics and mechanics were assessed in three loading conditions (i.e. one condition per trial, see details in the sections equipment/load characteristics and walking assessment below). Before starting the SMM at 12:00 am, the subjects checked their equipment and supplies.
The 21-h SMM was performed in a low- to middle-mountain environment close to the laboratory (see details in the section SMM characteristics and figure 1 below). After the SMM, the subjects immediately performed the post-SMM session (POST). POST began with an evaluation of instant RPF and rating of perceived exertion regarding the entire SMM (RPE). Then, after taking off their equipment, the subjects performed the NM function evaluation within 963 min after the SMM. Finally, the course of POST (i.e. unloaded standing gas collection and walking assessments on the treadmill in three loading conditions) was identical to that of PRE.
For logistical reasons (duration of PRE and POST, follow-up and safety of the subjects during the SMM, etc.), the subjects were divided into two subgroups (n = 5) and performed the protocol two days apart. Similarly, in order to avoid a period of rest between the end of the SMM and the NM function evaluations at POST, the subjects in a same subgroup performed the protocol at 20-min intervals.
Subject Characterization and Familiarization
During the inclusion sessions, the subjects were first examined by the same medical doctor that also performed anthropometric data collection. Stature was measured to the nearest 0.5 cm using a standardized wall-mounted height board. Leg length was measured from the great trochanter to the floor in a standing position with a tape measure. BM was measured to the nearest 0.1 kg with subjects standing in undergarments without shoes on a mechanical column weighing scale (Bascule type 286, Chollet, La Talaudie`re, France). BM Index was calculated as BM divided by height (in m) squared. Body fat percentage was estimated using skinfold thickness values and Durnin & Womersley standard equations [35]. Skinfold thickness values were measured to the nearest millimeter in triplicate at the biceps, triceps, subscapular, and suprailiac points on the left and right side of the body using a Harpenden skinfold caliper (British Indicators, West Sussex, UK).
At each of these four points, the mean value for the six skinfold thicknesses was calculated.
After this examination, the subjects performed an incremental running maximal test on a level treadmill. This test consisted of progressive 3-min stages separated by 1 min of rest. Running speed was 10 km.h
21at the first stage and the increment of speed was 1.5 km.h
21per stage until exhaustion. Heart rate (HR) was monitored continuously using a three-channel electrocardiogram (Cardiotest EK51, Hellige GMBH, Freiburg, Germany). Expired gases were collected during the last 30 s of each 3-min stage to determine oxygen consumption ( V VO _
2). The subjects breathed through a two-way nonrebreathing valve (series 2700; Hans Rudolph, Kansas City, MO) connected to a three-way stopcock leading to a 100-L Douglas bag. The volume of the expired gas
was measured by means of a Tissot spirometer (Gymrol, Roche-la- Molie`re, France), and fractions of expired gases were determined with a paramagnetic O
2analyzer (cell 1155B; Servomex, Crowborough, England) and an infrared CO
2analyzer (Normo- cap Datex, Helsinki, Finland). The analyzers were calibrated with mixed gases of known composition. HR
maxand V VO _
2maxcorre- sponded to the highest values obtained at steady state during the last running stage.
Particular attention was paid to familiarize the subjects with the devices/protocols used during the specific experimentation, especially the MVC and electrical stimulation of the KE and PF muscles. The subjects repeated trials of the procedures until the results were reproducible.
Equipment/load Characteristics
For a detailed description and illustration of the equipment, the reader should refer to a recent article from our research group [4].
Briefly, during the laboratory walking trials, three equipment conditions were tested, a Sportswear condition taken as reference (SP, mass #1 kg, corresponding to #1.4% of the subjects’ BM) and two configurations of the new French infantry combat system (FELIN, Sagem, France) without rifle: Battle equipment (BT, 22.461.1 kg, 27.261.9% BM) and Road March equipment (RM, 37.961.4 kg, 46.163.6% BM, corresponding to BT with an additional backpack).
During the SMM, both military BT and RM equipment were carried alternately (according to the military actions performed, e.g. RM while marching and BT while patrolling, see figure 1) with an additional demilitarized rifle weighting 5.01 kg (FA-MAS FELIN, France) to create conditions comparable to those found in the military theater. The total loads carried during the SMM were therefore 27.461.1 kg in BT and 42.961.4 kg in RM, corre- sponding to 33.462.6% BM and 52.264.2% BM respectively. BT and RM configurations were designed to represent and meet the common necessities of a prolonged patrol and reconnaissance mission. Each participant was familiarized with all of the equipment tested prior to the experiment.
Simulated Military Mission (SMM) Characteristics
An overview of the 21-h SMM design, intensity and integration
in the general protocol is given in figure 1. The SMM was
specifically designed by former officers to represent a typical
prolonged patrol and reconnaissance mission. It began with a 4-h
road march to simulate quietly approaching a hostile zone
represented by a fragment of forest located in a middle-mountain
environment 15 km away from the laboratory (uphill walking,
570 m of positive and 240 m of negative elevation change, mean
walking speed of ,4 km.h
21). On arrival near the hostile zone, the
subjects had a 1-h period to rest and ration. Each subject had a
French war ration (‘‘RCIR’’; 3200 kcal, protein: 13%, lipid: 32%,
carbohydrate: 55%) in his pack to provide sustenance over the
entire SMM. After this reconditioning period, the subjects moved
through the enemy forest (,30 min) in order to find and install
(,30 min) an advanced camp. The objective of the following 2-h
recon period was to progressively secure the zone that surrounded
the camp while approaching a strategic enemy lane for
observation. During the night hours, the subjects were equipped
with night vision devices. Moreover, during the recon periods of
the SMM, experimenters drove and moved through the forest to
simulate enemy activity. The subjects were also told to try and
intercept the experimenters as they tried to reach the advanced
camp, just as enemies would do. After the aforementioned 8 h of
various activities, the subjects were allowed two 3-h sleeping
periods separated by another 2-h recon period. Sleeping periods
were set and imposed according to the common turnover observed in missions and in previous reports [9]. Then one last 105-min recon was performed prior to a 15-min period during which the subjects gathered their equipment prior to walking back to the laboratory. The return road march was the exact reverse of the first one (downhill walking, 570 m of negative and 240 m of positive elevation change) except it was performed in 3 h because of the major negative slope (mean walking speed of ,5.5 km.h
21).
Fatigue Assessment: Neuromuscular Function
Experimental materials, preparation and setting. In the present study, we used the exact same (i) ergometers, positioning and instructions to subjects, (ii) electrical stimulation system and neural detection procedure, (iii) electrode models, electromyo- graphic recording system and skin preparation procedure as in a recent study from our research group [22].
Maximal voluntary contractions: the subjects were strongly encouraged during all the MVCs. For the KE testing, the subjects were seated in the frame of a Cybex II (Ronkonkoma, NY) and Velcro straps were strapped across the chest and hips to avoid lateral and frontal displacements. Subjects were also instructed to grip the seat during the MVC to further stabilize the pelvis. The KE muscular mechanical response was recorded with a strain
gauge (SBB 200 Kg, Tempo Technologies, Taipei, Taiwan) located at the level of the external malleolus. All measurements were taken from the subject’s right leg with the knee and hip flexed at 90 degrees from full extension. PF muscles were tested with an instrumented pedal (CS1060 300 Nm, FGP Sensors, Les Clayes Sous Bois, France). For the PF testing, the subjects were seated in the frame of a Cybex II similar to that used for KE. Velcro straps were also strapped across the chest and hips to avoid lateral and frontal displacements, and across the forefoot to limit heel lift during the MVC. The hip, knee and ankle angles were set at 90 degrees from full extension.
Electrical stimulation: after femoral (for KE) and posterior tibial nerve (for PF) detection with a ball probe cathode pressed into the femoral triangle and the popliteal fossa, respectively, electrical stimulation was applied percutaneously to the motor nerve via a self-adhesive electrode pressed manually (10-mm diameter, Ag- AgCl, Type 0601000402, Controˆle Graphique Medical, Brie- Comte-Robert, France). The anode, a 106 5 cm self-adhesive stimulation electrode (Medicompex SA, Ecublens, Switzerland), was located either in the gluteal fold (for KE) or on the patella (for PF). A constant current stimulator (Digitimer DS7A, Hertford- shire, United Kingdom) was used to deliver a square-wave stimulus of 1000- m s duration with maximal voltage of 400 V.
The stimulation intensity (70.6615.6 mA at PRE and Figure 1. Typical heart rate (HR) of a subject throughout the protocol. Figure includes the 21-h simulated military mission (SMM) and the pre-SMM (PRE) and post-SMM (POST) measurement sessions. Altitude, chronology and equipment conditions are inserted on and under the HR graph as indicative data. BT: battle equipment (27.461.1 kg corresponding to 33.462.6% of the subjects’ BM), RM: road march equipment (42.961.4 kg, corresponding to 52.264.2% BM).
doi:10.1371/journal.pone.0043586.g001
72.4616.9 mA at POST in KE, and 64.6617.2 mA at PRE and 65.3615.1 mA at POST in PF) was determined from maximal mechanical response to single twitch delivered to the relaxed muscle. This stimulation intensity was supramaximal and corre- sponded to 130% of the optimal intensity.
Electromyographic recordings: the EMG signals of the right vastus lateralis (VL) and soleus (SOL) were recorded using bipolar silver chloride surface electrodes of 10-mm diameter (Type 0601000402, Controˆle Graphique Medical, Brie-Comte-Robert, France) during the MVCs and electrical stimulation. The recording electrodes were taped lengthwise on the skin over the muscle belly, with an interelectrode distance of 25 mm. The position of the electrodes was marked directly on the skin with a permanent marker so that they could be placed in the exact same position before and after the SMM. The reference electrode was on the patella (for VL EMG) or malleolus (for SOL EMG). Low impedance (Z,5 kV) at the skin-electrode surface was obtained by abrading the skin with fine sand paper and cleaning with alcohol.
EMG data were recorded with PowerLab system (16/30 - ML880/P, ADInstruments, Bella Vista, Australia) with a sampling frequency of 2000 Hz. The EMG signal was amplified with octal bio-amplifier (Octal Bioamp, ML138, ADInstruments), with a bandwidth frequency ranging from 5 to 500 Hz (input impedan- ce = 200 MV, common mode rejection ratio = 85 dB, gain = 1000), transmitted to the computer and analyzed with LabChart 6 software (ADInstruments).
Experimental procedure. The present procedure was also identical to that used in our recent study [22]. Briefly, the NM function evaluation consisted of determining the isometric KE and PF MVC. During MVC, when the torque had reached a plateau, a high-frequency (100 Hz) doublet was superimposed on the contracted muscle. Finally, ,2 s after the end of the MVC, evoked stimuli consisting of a high-frequency (100 Hz) doublet, a low- frequency (10 Hz) doublet and a single twitch were delivered to the relaxed muscle in a potentiated state. This experimental set (MVC with superimposed doublet + evoked stimuli to the relaxed muscle) was repeated three times for both muscle groups with recovery of 1 min between repetitions.
Experimental variables and data analysis. M-wave: for both VL and SOL, M-wave peak-to-peak amplitude (in mV) and duration (in ms) were averaged from the EMG data from the three single potentiated twitches delivered to the relaxed muscle. M- wave characteristics provided information on action potential propagation.
Mechanical responses to nerve stimulation: for both KE and PF, the amplitude of the potentiated high-frequency doublet (PDb100), the ratio of paired-stimulus peak forces at 10 Hz to 100 Hz (Db10:100) and the amplitude of the potentiated peak twitch (Pt) were averaged from the values computed during the three experimental sets. Potentiated twitch contraction time (CT, in ms) and half-relaxation time (HRT, in ms) were also determined and were also calculated as the mean values of the three single twitches. Mechanical responses to nerve stimulation were used to determine the extent and origin of peripheral fatigue. In particular, Db10:100 was used to assess LFF [36].
Maximal voluntary contraction and maximal voluntary activa- tion level: the highest value of the three MVC was determined for both KE (in N) and PF (in Nm). MVC provided a global index of fatigue. Activation level (%VA) was calculated as follows:
%VA ~ (1 { Superimposed Db100=Potentiated Db100) | 100
%VA was used as an indicator of central fatigue. Finally, the root mean square (RMS) values of the VL and SOL EMG activity were calculated during the best MVC trial over a 0.5-s period after the MVC had reached a plateau and before the superimposed stimulus was delivered. This RMS value was then normalized to the maximal peak-to-peak amplitude of the M-wave to obtain RMS.M
21. The latter variable provided complementary infor- mation about central fatigue.
Fatigue Assessment: Perceived Fatigue and Global Exertion
RPE was measured at POST only using the 6 to 20 Borg scale [37]. The subjects were asked to quantify the exertion character- izing the entire SMM. RPF was measured at PRE and POST, using the same 6 to 20 Borg scale applied to the sensation of fatigue. The subjects were asked to quantify their instant sensation of general fatigue. RPE and RPF were administered individually to subjects by the same experimenter for all subjects.
Walking Assessment
The three equipment conditions, SP, BT and RM, were performed in a randomized and counterbalanced order, and trials were separated by 5 to 10 min during which subjects rested and changed their equipment. The 4 km.h
21walking speed was chosen for its economical feature in normal adult locomotion [3,38] and its consistency with the average walking speed currently used during military missions and experimentations [39]. Finally, rifle carriage was excluded to allow comparisons of our results with non-military studies since walking kinetics are altered by a limitation of the arm swing during rifle carriage [40]. Complete details of the physiological and biomechanical methods employed have been recently presented [4]. Thus, only the main principles and parameters are summarized below.
Walking Energetics
Energetic data were obtained from indirect calorimetry (Douglas bag method, vide supra). Expired gases were collected during the last 30 s of each 3-min walking trial. Unloaded standing metabolic rate and gross metabolic rate of walking (both in W) were determined from the steady-state V VO _
2and V VCO _
2using Brockway’s standard equation [41]. The metabolic rate measured during unloaded standing was subtracted from all gross walking values to compute the net metabolic rate (in W) [5]. Gross and net metabolic rates (in W) were divided by walking speed (in m.s
21) to obtain the gross and net energy costs of walking (C
W, in J.m
21).
Gross and net C
Wwere also divided by the total mass in motion on the treadmill (TM, in kg), i.e. subject plus equipment, to obtain mass-relative gross and net C
W(C
W.TM, in J.kg
21.m
21).
Walking Mechanics
Walking mechanics were analyzed using an instrumented 3-D force treadmill (ADAL, HEF Tecmachine, Andre´zieux-Bouthe´on, France) consisting of two left-right frames and belts allowing separate measurements of the left- and right-foot ground reaction forces (for complete description and validation, see ref. [42]).
Parameters were recorded over 20 s, 1.5 min after the beginning of each trial in order to ensure the stabilization of the gait pattern and avoid disturbances from the metabolic measurements. All data were sampled at 200 Hz and low-pass filtered at 30 Hz.
Mechanical analyses were performed over five consecutive strides,
one stride being defined as the period between two consecutive
right heel strikes. Mechanical parameters were computed for each
stride and then averaged to describe a typical mean stride.
Spatio-temporal parameters of walking were calculated from vertical ground reaction force signals. A duty factor (in %) was calculated as the ratio of stance duration to stride duration, and double support duration (in %) as the ratio of stance duration to stride duration. Finally, step frequency (in Hz) was computed as:
(stride duration/2)
21.
Kinetic parameters were computed from vertical, antero- posterior, and medio-lateral ground reaction forces. The external mechanical work (W
ext, in J.m
21), i.e. the work done by the muscles to lift and accelerate the center of mass (COM), was calculated according to Cavagna’s standard method [43]. W
extwas also normalized by the total moving mass (TM) to obtain mass- relative W
ext(W
ext.TM, in J.kg
21.m
21). The inverted pendulum recovery of mechanical energy of the COM (in %) was calculated according to Schepens et al. [44]. The internal work done during the double contact phase (W
int,dc, in J.m
21), i.e. the work done by one leg against the other during the transfer from one foot to the other, was calculated from the forces exerted by each lower limb on the ground measured separately, as proposed by Bastien et al.
[45]. W
int,dcwas also divided by TM to obtain mass-relative W
int,dc(W
int,dc.TM, in J.kg
21.m
21). Finally, locomotor efficiency (in
%) was calculated as the ratio of W
extplus W
int,dcto net C
W(all in J.m
21).
Statistical Analyses
All descriptive data are presented as mean 6 SD. Normal distribution of the data was checked by the Shapiro-Wilk normality test and variance homogeneity between samples was tested by the F-Snedecor test. When conditions of t-test and analysis of variance (ANOVA) application were respectively met, each variable studied was compared (i) between the different times of measurements (i.e. PRE vs. POST) using paired t-tests for the NM data, or (ii) in the three different conditions of equipment across times of measurement (time 6 equipment) using two-factor within subjects ANOVAs for the data of locomotion. Newman- Keuls multiple comparison post-hoc tests were used to determine between-means differences if the ANOVA revealed a significant main effect. For the few NM variables that did not meet normality (i.e. KE MVC, SOL M-wave peak-to-peak duration, and PF
%VA), Wilcoxon tests were used. Statistical significance was accepted at P,0.05. Effect size was calculated using Cohen’s d by dividing the mean difference between PRE and POST (in absolute value) by the between-subject standard deviation at PRE [46].
Effect size was therefore used and considered as a supplementary index of the importance of the effect for the variables showing significant differences or statistical trends between PRE and POST. A Cohen’s d value of 0.2 was considered as a small effect, a value of 0.5 was considered as a moderate effect, and a value of 0.8 was considered as a large effect [46].
Results
Heart Rate, Global Exertion and Perceived Fatigue The mean HR over the first 4-h road march was 139618 bpm (which corresponded to 73.167.3% of HR
max) and ranged from 90617 to 171617 bpm (47.067.5% to 90.166.6% of HR
max).
Over the 14-h ‘‘battle’’ phase, i.e. the entire SMM duration (21 h) minus the 7 hours (4 h + 3 h) of road marches, it was 91616 bpm (45.364.9% of HR
max) and ranged from 55611 to 156613 bpm (26.463.9% to 82.865.7% of HR
max). Finally, over the 3-h return road march, the mean HR was 114617 bpm (59.765.6% of HR
max) and ranged from 80619 to 149615 bpm (41.867.7% to 78.366.6% of HR
max). The exertion (RPE) represented by the entire SMM was 16.762.4 on the 6–20 Borg scale, which
corresponded to a ‘‘very hard effort’’. Individual RPE values ranged from 13 (n = 1, ‘‘somewhat hard effort’’) to 20 (n = 2,
‘‘exhausting effort’’). Last, the instant-sensation of general fatigue (RPF) was significantly increased from PRE to POST (P,0.01, d = 3.47). RPF were 8.362.2 (corresponding to ‘‘very very low fatigue’’) and 15.962.1 (‘‘very high fatigue’’) at PRE and POST, respectively.
Neuromuscular Fatigue and its Central and Peripheral Components
As shown in figures 2 and 3, MVC declined significantly by 210.263.6% for KE (P,0.01, d = 0.50) and a strong trend (210.7616.1%) was observed for PF (P = 0.06, d = 0.82) after the SMM. Concerning the peripheral aspect of fatigue induced by the SMM, PDb100 decreased significantly for KE (26.3968.01%, P,0.05, d = 0.43) and even more for PF (218.269.8%, P,0.001, d = 1.14). A trend towards Db10:100 decline (i.e. LFF index) was observed for KE (25.4669.35%, P = 0.08, d = 0.53) but not for PF (P = 0.21) after the SMM. There was no significant correlation between KE and PF changes in MVC or PDb100. Table 1 shows the characteristics of the mechanical (KE and PF) and EMG (VL and SOL) responses to single electrical stimuli of the femoral and tibial motor nerves to the relaxed muscle. In particular, this table shows that the SMM induced considerable effects on the potentiated Pt and CT for both KE and PF. Concerning the central aspect of fatigue, the SMM did not induce change in %VA for PF (P = 0.73), but a decreasing trend was observed for KE
Figure 2. Neuromuscular parameters measured in the knee extensors (KE) before (PRE) and after (POST) the mission. a.
Maximal voluntary contraction (MVC); b. Potentiated high-frequency doublet (PDb100); c. Ratio of paired stimulation peak forces at 10 Hz to 100 Hz (Db10:100); d. Voluntary activation level (%VA). *P,0.05,
**P,0.01.
doi:10.1371/journal.pone.0043586.g002
(22.1862.96%, P = 0.05, d = 0.96). RMS.M
21did not change significantly from PRE to POST for either KE (from 6.8962.81%
to 6.1762.01%, P = 0.14) or PF (from 3.5860.63% to 3.5861.09%, P = 0.99).
Walking Energetics and Mechanics
Metabolic and mechanical parameters of walking, taking into account the main effects of SMM-related fatigue, acute load carriage and their interaction are presented in tables 2 and 3, respectively. ANOVAs showed that acute military equipment (i.e.
load) carriage induced significant increases in both gross and net C
W(P,0.0001). A trend toward increased net C
W.TMwas also observed when load increased (P = 0.098) while gross C
W.TMdecreased significantly in this condition (P = 0.012). Concerning the mechanics of walking, all spatio-temporal parameters were altered by acute load carriage (all P,0.05) and both absolute and mass-relative W
int,dcand W
extincreased (all P,0.01). However, the inverted pendulum recovery of mechanical energy and the locomotor efficiency did not change while carrying loads. In contrast to these acute effects of load carriage, neither the time between PRE and POST (i.e. the fatigue induced by the SMM), nor the interaction between time and equipment (i.e. fatigue 6 load) had a significant effect on the parameters describing subjects’
walking energetics and mechanics.
Discussion
The Main Purpose of the Present Study was to Investigate the combined effects of heavy load carriage and exercise of extreme duration on NM fatigue. Specifically, we hypothesized that both central and peripheral NM function changes due to a 21-h military mission would be higher than after walking with load carriage for intermediate durations, i.e. up to ,3 h [18,19], but lower (especially at the peripheral level) than after ultra-marathon runs lasting up to ,40 h and inducing extensive muscular damage [21,22]. The main results of this study are: (i) MVC declined by ,10% after the SMM and the origin of fatigue was essentially Figure 3. Neuromuscular parameters measured in the plantar
flexors (PF) before (PRE) and after (POST) the mission. a.
Maximal voluntary contraction (MVC); b. Potentiated high-frequency doublet (PDb100); c. Ratio of paired stimulation peak forces at 10 Hz to 100 Hz (Db10:100); d. Voluntary activation level (%VA). ***P,0.001.
doi:10.1371/journal.pone.0043586.g003
Table 1. Potentiated peak twitch of knee extensors (KE) and plantar flexors (PF), and M-wave characteristics of vastus lateralis (VL) and soleus (SOL) muscles, before (PRE) and after (POST) the Simulated Military Mission.
Parameters and muscles PRE POST t-test Pvalues % Change Cohen’s d
Potentiated peak twitch
KE (N) 180632 160635
,0.01 210.969.6 0.60PF (Nm) 25.665.6 20.964.6
,0.001 218.3±9.5 0.84Contraction time
KE (ms) 10664 10068
0.07 25.869.2 1.48PF (ms) 94.3616.6 82.868.1
,0.01 210.9±9.2 0.69Half relaxation time
KE (ms) 89.7618.2 84.0617.4 0.53
23.6623.10.31
PF (ms) 109614 100618 0.17
27.7618.80.68
M-wave peak-to-peak amplitude
VL
(mV) 11.963.7 12.163.1 0.63
+4.8615.60.06
SOL
(mV) 7.862.9 9.262.6
,0.01 +22.2±21.2 0.48M-wave peak-to-peak duration
VL
(ms) 9.862.3 9.861.4 0.89
+1.4613.00.03
SOL
(ms) 3.161.2 3.161.4 0.99
21.2613.20.00
Data are presented as mean
6SD.
doi:10.1371/journal.pone.0043586.t001
peripheral for both KE and PF, both of which are major power generators in human locomotion, (ii) a trend toward moderate LFF was detected for KE muscles, suggesting the presence of muscular mechanical and metabolic disturbances, and (iii) NM function changes were concomitant with severe subjective fatigue but the latter was not coupled with the central NM mechanisms.
The secondary aim of this study was to investigate the effects of extreme-duration heavy load carriage on the energy cost, mechanical work and spatio-temporal pattern of walking. In particular, we hypothesized that the fatigue induced by a 21-h SMM would affect the biomechanics of walking. The results indicated that neither walking energetics, nor walking mechanics were altered by SMM-related fatigue. Taken together, these results bring the first insight into the physiological and biomechanical consequences of exercises of extreme duration with heavy load carriage. Overall, it appears that extreme-duration heavy load
carriage induced modest consequences in experienced walkers/
carriers, contrary to what was expected from both NM and locomotor standpoints.
Exertion and Perceived Fatigue Due to the Military Mission
HR data monitored over the entire SMM showed that the most severe period of the mission was the first 4-h march (,7367% of HR
max), very likely due to the long and sometimes steep uphill sections (figure 1) with heavy load carriage (,43 kg). In contrast, the 14-h-battle phase was the least demanding period of the SMM (,4565% of HR
max), notably because of the two 3-h sleeping periods permitted during this phase. Nevertheless, HR peaked around 8366% of HR
maxduring the battle phase, showing that the recons and patrols were quite intense. Finally, although the 3-h return march was performed at a high speed (,5.5 km.h
21on Table 2. Energy cost of walking in Sport (SP), Battle (BT) and Road March (RM) conditions, before (PRE) and after (POST) the Simulated Military Mission.
PRE POST ANOVAPvalues
Parameters SP BT RM SP BT RM T E T6E
Gross
CW(J.m
21) 271636
£336658 369655 278648
#337664 355668 0.283
,0.00010.529
Gross
CW.TM(J.kg
21.m
21) 3.3060.47 3.2260.50 3.0760.38 3.4360.54
$3.2860.60 3.0060.50 0.121
0.0120.484
Net
CW(J.m
21) 144637
£205656 239670 150642
#210656 228660 0.578
,0.00010.529
Net
CW.TM(J.kg
21.m
21) 1.7460.44 1.9760.46 1.9860.44 1.8560.45 2.0460.52 1.9260.45 0.303
0.0980.596 Values are presented as mean
6SD. ANOVA
Pvalues represent the main effects of time (T) and equipment (E), and the interaction effect (T6 E). Post-hoc results are presented as:
£
SP, BT
,RM;
#
SP, BT = RM;
$
SP. RM.
CW
: energy cost of walking;
CW.TM: energy cost of walking normalized to the total mass (TM) moving on the treadmill, i.e. subject plus equipment.
doi:10.1371/journal.pone.0043586.t002
Table 3. Spatio-temporal and kinetic parameters of walking in Sport (SP), Battle (BT) and Road March (RM) conditions, before (PRE) and after (POST) the Simulated Military Mission.
PRE POST ANOVAPvalues
Parameters SP BT RM SP BT RM T E T6E
Step Frequency (Hz) 1.7360.09
$1.7360.07 1.6960.07 1.7560.09 1.7660.10 1.7160.07 0.147
0.0120.864
Duty factor (%) 64.960.7
£66.360.8 66.960.9 65.160.4
£66.460.8 67.461.2 0.211
,0.00010.299
Double support duration (%) 29.661.4
£32.661.6 33.861.7 30.360.9
£32.661.7 35.062.3 0.196
,0.00010.140
Wext
(J.m
21) 19.463.6
£28.564.8 31.967.0 19.263.3
£26.664.1 31.465.3 0.117
,0.00010.187
Wext.TM
(J.kg
21.m
21) 0.22860.041
#0.26760.041 0.26160.060 0.23160.046
#0.25460.038 0.26260.050 0.557
0.0040.238
Recovery (%) 72.263.9 70.863.1 71.563.9 71.964.6 72.362.8 71.763.0 0.496 0.874 0.174
Wint,dc
(J.m
21) 11.562.8
£15.964.7 19.765.0 11.263.1
£15.263.9 19.964.4 0.695
,0.00010.587
Wint,dc.TM
(J.kg
21.m
21) 0.13460.030
£0.14760.041 0.16160.041 0.13360.032
&0.14460.035 0.16660.038 0.957
,0.00010.559
Locomotor efficiency (%) 22.667.9 23.9610.8 22.968.9 19.864.4 20.766.6 23.967.9 0.225 0.362 0.300
Values are presented as mean
6SD. ANOVA
Pvalues represent the main effects of time (T) and equipment (E), and the interaction effect (T6 E). Post-hoc results are presented as:
$
SP = BT
.RM;
£
SP, BT
,RM;
#
SP, BT = RM;
&
SP = BT
,RM.
Wext
: external mechanical work;
Wext.TM: external mechanical work normalized to the total moving mass (TM), i.e. subject
+equipment; Recovery: fraction of mechanical energy of the center of mass recovered via the inverted pendulum mechanism;
Wint,dc: mechanical work done by one leg against the other leg during double contact;
Wint,dc.TM
